Although the development of tolerance to organophosphorus inhibitors of cholinesterase (ChE) activity, commonly used as insectides, has been recognized the mechanism by which it develops is not well understood. The organophosphorus compounds vary so widely in their affinities for different types of ChE, their distribution in the body, and their biotransformations that the findings with one agent cannot be held generally applicable to the group. It is, therefore, not justifiable to assume that an adaptation to one anticholinesterase agent or occurring in one species is generally transferable to other agents or species. It is proposed that the development of tolerance is the consequence of drug induced alterations of the steady state level of the cholinergic transmitter, acetylcholine (ACh). This reduction in ACh is secondary to depression of its synthesis, its storage or release from cholinergic synaptic terminals and decreased sensitivity of the receptor sites to the transmitter. However, the possibility that an increased rate of ChE production or an induction of detoxifying enzymes may be contributing factor in such adaptations will not be overlooked. The purpose of this investigation is to search for a biochemical mechanism of organophosphate tolerance using the following approaches: (1) a study of the possible relationship between synthesis, storage and release of ACh and sensitivity of the cholinergic receptor to ACh. Tissue from tolerant animals will be routinely used. (2) An investigation of the role of detoxifying mechanisms in the development of tolerance. It is hoped that information obtained from these studies will help reveal the mechanism of tolerance to O-P agents. These studies should lead to a better understanding of the biochemical control of tolerance and may provide new approaches to the understanding of actions of insecticides and other chemical environmental hazards.